1. Field of the Invention
The present invention relates to a color cathode ray tube apparatus, and more particularly, to a color cathode ray tube apparatus having an electron gun assembly, in which three electron beams arranged in line are focused and converged by means of a large-aperture electron lens common to the beams.
2. Description of the Related Art
FIG. 1 shows a conventional color cathode ray tube apparatus. Color cathode ray tube apparatus 1 comprises envelope 11 which includes panel section 2, funnel section 8 bonded to panel section 2, and neck section 10 continuous with funnel section 8. Panel section 2 has which is substantially rectangular face plate 4 and skirt 6 extending from the peripheral edge of plate 4. The inside of the color cathode ray tube is kept at a vacuum by sections 2, 8 and 10. Electron gun assembly 12 is used for emitting three electron beams B.sub.R, B.sub.G, and B.sub.B and is housed inside neck section 10. Deflecting device 14 is mounted on the outer peripheral surfaces of funnel and neck sections 8 and 10 respectively. The deflecting device serves to generate magnetic fields in order to deflect electron beams B.sub.R, B.sub.G, and B.sub.B horizontally and vertically. Phosphor screen 16 is formed on the inner surface of face plate 4 of panel section 2. Inside the tube, shadow mask 18, which is substantially rectangular in shape, is arranged opposite screen 16 so that a predetermined space is kept between mask 18 and face plate 4. Mask 18, which is formed of a metal sheet, has a number of perforations 20. Internal conductor film 22 is applied to the inner wall surface of a boundary portion between funnel and neck sections 8 and 10, while external conductor film 24 is applied to the outer wall surface of funnel section 8.
Three electron beams B.sub.R, B.sub.G, and B.sub.B emitted from their corresponding electron guns of electron gun assembly 12 are deflected by means of deflecting device 14. The deflected beams are converged in the vicinity of perforations 20 of shadow mask 18. Converged in this manner, electron beams B.sub.R, B.sub.G, and B.sub.B land on specific regions of phosphor screen 16 which glow with three colored lights, red, green, and blue, respectively. Thus, beams B.sub.R, B.sub.G, and B.sub.B from assembly 12 cause screen 16 to glow with red, green, and blue lights, respectively.
Electron gun assembly 12 includes electron beam forming unit GE for generating, accelerating, and controlling electron beams B.sub.R, B.sub.G, and B.sub.B to be emitted in line, and main electron lens unit ML for focusing and converging the electron beams. Electron beams B.sub.R, B.sub.G, and B.sub.B are deflected by deflecting device 14 to be used to scan phosphor screen 16, thus forming a raster.
There are some conventional methods for converging three electron beams. One of these methods is disclosed in U.S. Pat. No. 2,957,106, in which an electron beam emitted from a cathode is initially skewed before it is converged. In another method disclosed in U.S. Pat. No. 3,772,554, electron beams are converged in an arrangement such that two outside openings, out of three openings in an electrode of an electron gun, are slightly outwardly eccentric to the central axis of the electron gun.
The deflecting device includes a horizontal deflecting coil, for horizontally deflecting the electron beams, and a vertical deflecting coil, for vertically deflecting the electron beams. When the three electron beams are deflected by means of the deflecting device, in the conventional color cathode ray tube apparatus, they cannot be accurately converged on the phosphor screen. Therefore, some measures have been taken to converge the electron beams accurately. Among these measures, there is a method called a convergence-free system, in which horizontal and vertical deflecting magnetic fields are generated in the forms of a pincushion and a barrel, respectively, whereby the three electron beams are converged.
In this convergence-free system, the electron beams suffer deflective aberration produced by the pincushion-type horizontal deflecting magnetic field. At a horizontal end portion of the screen, therefore, spots of the electron beams suffer halos. Thus, the picture quality is considerably lowered.
Large-sized color cathode ray tube apparatuses of high quality have recently been coming into wide use. These apparatuses, however, have the following problems.
(1) The diameter of beam spots on the phosphor screen.
(2) Distortion of the beam spots on the peripheral region of the phosphor screen caused when the electron beams are deflected.
(3) Convergence of the electron beams on the whole surface of the phosphor screen.
In the large-sized color cathode ray tube apparatuses, the distance from the electron gun to the phosphor screen is long, so that the electrooptical magnification of an electron lens is high. Accordingly, the diameter of the beam spots on the phosphor screen is so long that the resolution is low. Thus, in order to reduce the spot diameter, the performance of the electron lens of the electron gun must be improved.
In general, the main electron lens unit is arranged so that a plurality of electrodes, each having apertures, are coaxially arranged, and a predetermined voltage is applied to each of the electrodes. Electrostatic lenses, such as the main electron lens unit, may be classified into several types, depending on the electrode configuration. Basically, the lens performance can be improved by forming a large-aperture lens with large electrode apertures, or by lengthening the distance between the electrodes to change the potential slowly, thereby forming a long-focus lens.
In the color cathode ray tube apparatuses, however, the electron gun is housed inside a neck, formed of a slender glass cylinder, so that the diameter of the electrode aperture, i.e., lens aperture, is physically restricted. Also, the distance between the electrodes is limited, in order to prevent converging electric fields formed between the electrodes from being influenced by other electric fields inside the neck.
In the color cathode ray tube apparatuses of a shadow-mask type, in particular, three electron guns are arranged in a delta or in-line configuration. If space Sg between electron beams from the electron guns is short, the three beams can be easily converged on the phosphor screen, so that power supply to the deflecting device can be reduced. Therefore, three conventional electron lenses arranged on the same plane are made perfectly to overlap one another, thereby forming one large-aperture electron lens. The best electron lens performance can be obtained with use of the large-aperture electron lens. FIG. 2 shows an example of the large-aperture electron lens. Although the core of each electron beam is small, in this example, the entire electron beam is not small enough. When three electron beams B.sub.R, B.sub.G, and B.sub.B, arranged at spaces Sg, pass through common large-aperture electron lens LEL, outside beams B.sub.R and B.sub.B are excessively converged and focused if central beam B.sub.G is properly converged. Further, outside beams B.sub.R and B.sub.B suffer a substantial coma, so that spots SP.sub.R, SP.sub.G, and SP.sub.B of the three electron beams cannot be superposed, and outside spots SP.sub.R and SP.sub.B are distorted. The three electron beams can be properly converged to reduce the coma by shortening beam space Sg to some degree, depending on lens aperture D of electron lens LEL. In order to converge the three electron beams accurately on the phosphor screen, however, space Sg must be made very short. In the mechanical arrangement of an electron beam generating section, space Sg can be reduced only slightly.
FIG. 3 shows an electron gun disclosed in U.S. Pat. Nos. 3,448,316 or 4,528,476, as a means for solving the above problem. In this electron gun, the outside electron beam, out of three electron beams, is inclined at angle .theta. to a central beam as the beams are incident on electron lens LEL. The three electron beams intersect one another so as to pass through the central portion of lens LEL, whereby the convergence of the beams is suitably adjusted. Thereafter, the diffusing outside electron beams are deflected in opposite direction at angle .phi. by means of second lens LEL2, so that the three electron beams are converged on the phosphor screen. Thus, the convergence and focusing of the electron beams are improved in reliability. Nevertheless, the problem of the outside electron beams suffering the deflective aberration and coma is not solved yet.
A method for preventing over concentration of electron beams is described in Japanese Patent Application No. 62-186528. In order to converge the electron beams as shown in FIG. 4A, a plate member, as shown in FIG. 4B, is disposed on the side of an electron beam generating section, in the vicinity of a large-aperture electron lens of an electron gun. The plate member has a noncircular aperture common to the three electron beams. In this method, the three beams are rendered incident on the large-aperture electron lens without intersecting one another.
Since the plate member, however, has the common aperture for the passage of the three electron beams, according to the method described above, the electron beams cannot be properly focused if the convergence characteristic provided by the large-aperture electron lens is corrected. Accordingly, spots of the electron beams suffer a substantial coma. Thus, it is very difficult to control the three electron beams by means of the common large-aperture electron lens through which the electron beams pass.